1. Field of the Invention
This invention relates to a process of post-treating metal coated substrates, and to the composition for treating metal coated substrates to improve the adhesion-bonding, abrasion, and corrosion-resistant properties of the coated substrates. More specifically, this invention relates to a novel composition, and to the process of using the composition to post-treat metal coated substrates. The composition comprises an acidic aqueous solution comprising effective amounts of at least one water-soluble trivalent chromium salt, an alkali metal hexafluorozirconate, at least one tetrafluoroborate and/or hexafluorosilicate and effective amounts of water-soluble thickeners and/or water-soluble surfactants.
2. Description of Prior Art
Current high-performance post treatments for metal coated substrates are based on hexavalent chromium chemistry. Hexavalent chromium is highly toxic and a known carcinogen. As a result, the solutions used to deposit post-treatment coatings and the coating, per se are toxic. These coatings do, however, yield outstanding paint adhesion and corrosion resistance to the base metal. Typically, post-treatments or seal coatings are deposited onto the metal coating at about elevated temperatures and usually applied by immersion or spray processes. Post treatments are usually specified by the military or commercial specifications that govern each metal coating being treated. As such, there is not a unique xe2x80x9cpost treatmentxe2x80x9d specification for all metal coatings as there is for xe2x80x9cconversion coatedxe2x80x9d aluminums.
Further, environmental laws, executive orders, and local occupational, safety, and health (OSH) regulations are driving military and commercial users in the search for chromate-free post treatments. In the case of coated metals, the metal substrates and the coatings per se are relatively non-toxic. With the addition of a chromate post treatment, however, these coatings become toxic. In addition, the use of chromate post treatments is becoming more expensive as regulations tighten. Costs are becoming prohibitive with future restrictions imposed by the EPA. Moreover, certain processes like spraying chromate coating solutions are forbidden at some facilities due to OSH risk, thereby forcing the use of less-than-optimum solutions. Thus, while existing chromate post treatments are outstanding in their technical performance in that they provide enhanced corrosion protection and adhesion bonding e.g. paint and other coating at a low application cost, from a life-cycle cost, environmental, and OSH perspective, chromate coatings are detrimental for people and the environment.
This invention relates to trivalent chromium post treatment (TCP) compositions and to the processes for improving the adhesion and corrosion resistant properties of metal coated substrates. These coatings and processes are generally known as xe2x80x9cpost-treatmentsxe2x80x9d. Post-treatments are coatings in contact with the metal being treated after formation of the initial coating and therefore the post-treatments normally have no direct contact with the underlying substrate, except possibly through some pores in the metal coating.
More specifically, this invention relates to a composition and to the process of using said composition to post-treat metal coated substrates at ambient temperatures or higher e.g. temperatures ranging up to about 200xc2x0 F. Specifically, this invention relates to a composition for post-treating metal coatings to improve the corrosion-resistance and adhesion bonding properties thereof e.g. paint adhesion etc. The composition comprises an acidic aqueous solution having a pH ranging from about 2.5 to 4.5 and preferably 3.7 to 4.0, and comprises, per liter of said solution, from about 0.01 to 22 grams of a water-soluble trivalent chromium salt, about 0.01 to 12 grams of an alkali metal hexafluorozirconate, about 0.01 to 12 grams of at least one fluoro-compound selected from the group consisting of an alkali metal tetrafluoroborate, an alkali metal hexafluorosilicate and various combinations thereof, from about 0 to 10 grams per liter and preferably 0 to 2.0 grams of at least one water-soluble thickener, and from 0 to 10 grams per liter and preferably 0 to 2.0 grams of at least one water-soluble non-ionic, cationic or anionic surfactant.
It is therefore an object of this invention to provide an aqueous solution comprising a trivalent chromium salt, an alkali metal hexafluorozirconate, and effective amounts of a tetrafluoroborate and/or hexafluorosilicate for post-treating metal coated substrates to improve adhesion bonding and the corrosion resistance properties thereof
It is another object of this invention to provide a stable acidic aqueous solution having a pH ranging from about 2.5 to 4.5 containing a trivalent chromium salt for treating metal coatings.
It is a further object of this invention to provide a stable acidic aqueous solution containing trivalent chromium having a pH ranging from about 3.7 to 4.0 for treating metal coated substrates at about room temperature wherein said solution contains substantially no hexavalent chromium.